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Starry mix-up

By Charles Seife

THE BIZARRE antigravity force called the cosmological constant, which some
astronomers think is driving the Universe apart (see p 19), may be a mere
accounting error.

That’s the warning from astronomers who have noticed a new source of bias in
tracking supernovae. While the bias does not necessarily affect arguments for a
cosmological constant, it suggests that astronomers have probably not yet
pinpointed every possible source of error.

Astronomers announced last year that the cosmological constant would make the
Universe expand for ever, after they observed that very distant exploding stars
were much fainter than expected.

The significance of this finding was thrown into question when astronomers
discovered that the most distant of these supernovae might be inherently dimmer(New Scientist, 17 July, p 4). Now Dale Howell, a graduate student at the
University of Texas at Austin, points out another possible problem in a paper to
appear later this year in The Astrophysical Journal.

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Howell says that the central region of galaxies on older photographic plates
are usually overexposed, so supernovae are less likely to be discovered there.
In contrast, more modern digital photography exposes galaxies evenly.

This is a problem because supernovae in the centre of galaxies vary much more
in brightness than those on the edge. This leads to a selection bias: closer
supernovae discovered via photography might have different properties from
faraway supernovae found by more modern methods.

“It makes it easier still to imagine that there’s some systematic error,”
says Adam Riess, a supernova spotter at the University of California at
Berkeley. However, Riess says that he and his team have attempted to allow for
such a bias in their measurements.